Abstract
Ti(C, N)-Mo2C-Ni cermet as alternative materials was explored for use in alkaline conditions, replacing the WC-Co cemented carbides, since Co is classified as a potentially carcinogenic substance and there is potential hazard of “hard metal disease” under the exposure to cobalt dust. The changes in microstructure, corrosion rate and volumetric loss rate of the two materials were compared under electrochemical corrosion and erosion-corrosion in alkaline environment. The results demonstrates that Ti(C, N)-Mo2C-Ni cermet undergoes passivation when exposed to electrochemical corrosion of NaOH solution, resulting in a significant increase in oxygen content on the corroded surface. The corrosion rate of cermet is approximately one order of magnitude lower than that of the cemented carbide. Under the erosion-corrosion of an alkaline sand-water mixture, both the cermet and cemented carbide experience a gradual increase in volumetric loss rate with prolonging the erosion time. During erosion, the rim phase in cermet is fragile, so cracks easily penetrate it while the core phase remains intact. The medium-grained cemented carbide commonly demonstrates transgranular fracture mode, while in the fine-grained cemented carbide, cracks tend to propagate along phase boundaries. The erosive wear and damage caused by sand particles play a predominant role in the erosion-corrosion process of alkaline sand-water mixtures. This process represents an accelerated destructive phenomenon influenced and intensified by the combined effects of corrosion and erosion. It is confirmed that using cermet as an alternative anti-wear material to cemented carbides is feasible under alkaline conditions, and even better.
Industrial wastewater treatment has gained increasing attention due to its association with environmental protection and resource recyclin
Ceramic materials possess excellent corrosion resistance and wear resistance. However, when subjected to high-speed impacts from sand particles in wastewater, ceramic materials tend to fracture and fail due to their inherent brittleness. To address this limitation, high-toughness metal was incorporated into ceramics to improve the toughness and resistance to fracture. Among ceramic-based composite materials, WC-Co cemented carbide, mainly used as cutting tool materia
Despite its desirable characteristics, WC-Co cemented carbide relies on strategic scarce resources such as tungsten (W) and cobalt (Co). Moreover, Co has the potential to cause various health concerns, such as DNA strand breaks, chromosomal abnormalities, micronucleus formation, or sister chromatid exchanges, making it a potentially carcinogenic substanc
In the present study, the electrochemical corrosion and erosion-corrosion behavior of Ti(C, N)-based cermet and WC-Co cemented carbides were compared in alkaline conditions. The corrosion morphology, corrosion rates, microstructure, and volumetric loss rates of the materials in alkaline electrochemical were characterized. The objective is to provide valuable insights for selecting appropriate wear-resistant materials in alkaline conditions.
In this study, commercially available materials were used for the preparation of Ti(C, N)-Mo2C-Ni cermet and WC-Co cemented carbides. The raw materials included TiC0.7N0.3 powder, Mo2C powder, Ni powder, medium-sized WC powder, fine-sized WC powder, and Co powder.
| Raw powder | FSSS particle size/μm | Composition/wt% | ||||||
|---|---|---|---|---|---|---|---|---|
| Total carbon | Free carbon | O | N | Fe | Ni | Cr | ||
| TiC0.7N0.3 | 3.36 | 13.29 | - | 0.54 | 7.29 | 0.065 | - | - |
| Mo2C | 1.55 | 6.03 | 0.2 | 0.52 | 0.15 | - | - | - |
| Ni | 2.65 | 0.1 | - | 0.1 | - | 0.005 | - | - |
| WC60 | 6.44 | 5.96 | 0.02 | 0.02 | - | - | - | - |
| WCR10 | 1.2 | 5.95 | 0.02 | 0.1 | - | 0.012 | - | 0.41 |
| Co | 1.10 | 0.014 | - | 0.41 | - | - | 0.0014 | - |
Both the cermet and the cemented carbides were prepared by powder metallurgy techniques. The mixed powders were ground in a 1 kg stainless steel ball milling tank by a roll ball mill. For CN16, gasoline was used as the grinding medium, and for YG6 and YG6X, anhydrous ethanol was used with a dosage of 300 mL each time. The grinding media consisted of Φ6 mm WC-6Co alloy balls, with a ball-to-powder ratio of 5:1. The grinding speed was set at 56 r/min, and the grinding time was 72 h for CN16 and fine-sized YG6X, and 48 h for medium-sized YG6. After grinding, the slurry was filtered through a 38 μm sieve and allowed to settle for approximately 12 h. The remaining mixture was then dried in an electric heating vacuum drying cabinet at 90–100 °C for 50–60 min. The resulting powder was mixed with SD rubber binder, dried, and sieved through an 180 μm sieve. The binder ratios were 220 mL/kg for CN16 and 80 mL/kg for YG6 and YG6X. Subsequently, the mixed powders were pressed into green compacts with dimensions of 20 mm×6.5 mm×5.25 mm, and pre-sintered in a vacuum furnace at 1420 °C for YG6 and YG6X, and 1445 °C for CN16, with a holding time of 1 h. Pressure sintering was then performed under an argon atmosphere with a pressure of 5 MPa. The sintering temperature was 1400 °C and the holding time was 40 min to reduce the porosity of the vacuum-sintered bodies.
| Material | APS/μm | Density/g·c | TRS/MPa | Hardness/HRA |
|---|---|---|---|---|
| CN16 | 0.9 | 5.68 | 1714 | 92.8 |
| YG6 | 1.4 | 14.95 | 1763 | 91.8 |
| YG6X | 0.7 | 14.85 | 2185 | 93.2 |
Electrochemical corrosion tests were conducted on a CS4 three-electrode electrochemical workstation. The auxiliary electrode was Ag/AgCl, and the corrosion solution was a 5wt% NaOH solution. The dynamic potential scanning technique was employed with a scanning range from -2.0 V to 1.5 V, scanning rate of 2 mV/s, frequency of 2.00 Hz, and scanning time of 1500 s, at an experimental temperature of 25 ℃.
Fig.1 Schematic of the device for erosion-corrosion
Surface morphologies of the electrochemical corrosion and erosion-corrosion were examined using a Hitachi S-4800 high-resolution field emission scanning electron microscope (SEM) with secondary electron imaging. The surface composition was analyzed by energy dispersive spectrometer (EDS) attached to the SEM. In this study, the eroding corrosion evaluation focused on the volume loss rate to eliminate the density factor, which is significant because the density of CN16 is less than half of that of YG6 and YG6X. The volume loss rate was calculated by the following formula:
| (1) |
where the erosion-corrosion rate is represented as v (m
Fig.2 Surface morphologies of materials before (a, c, e) and after (b, d, f) electrochemical corrosion in alkaline solution: (a–b) CN16; (c–d) YG6; (e–f) YG6X
Fig.3 XRD patterns of surfaces after electrochemical corrosion: (a) CN16, (b) YG6, and YG6X
The primary hard phase in cemented carbides is WC, which exhibits sharp diffraction peaks in the XRD pattern of
| Ni+2O | (2) |
| Ni(OH)2+O | (3) |
Fig.4 EDS results of surface composition of materials after electrochemical corrosion in alkaline solution: (a) CN16; (b) YG6; (c) YG6X
When the metal binder Ni is corroded, the enclosed ceramic hard phase becomes exposed to the corrosive solution, leading to corrosion of the ceramic hard phase. In an alkaline electrochemical corrosion solution, the ceramic hard phase Ti(C, N) undergoes corrosion and oxidation, resulting in the formation of TiCNO, TiO, or TiO2. Moreover, the introduced Mo2C, which is dissolved in Ti(C, N), is also corroded and oxidized. The reaction equation
| TiCN+H2O=TiCNO+2 | (4) |
| 2TiCN+6H2O=2TiO+2CO2+N2+12 | (5) |
| 2TiCN+8H2O=2TiO2+2CO2+N2+16 | (6) |
| (7) |
| (8) |
The dissolution of the reaction product MoO3 in the corrosive solution leads to a decrease in corrosion resistanc
| Co+2O | (9) |
| Co(OH)2+O | (10) |
Fig.5 Polarization curves of electrochemical corrosion in alkaline solution
CN16 exhibits a typical passivation phenomenon during electrochemical corrosion in alkaline solution. As the potential increases from 0 V, the current density also increases. Once the potential reaches its peak value, the current density rapidly decreases. This decrease occurs due to the formation of a high-resistance passivation film on the surface of CN16, which improves its corrosion resistanc
| (11) |
| Material | R value | ba/mV·de | bc/mV·de | Ecorr/V | Rp/Ω·c | Icorr/mA·c | Corrosion rate/mm· |
|---|---|---|---|---|---|---|---|
|
CN16 YG6 YG6X |
0.98 0.99 1.00 |
142.13 122.98 129.18 |
54.51 57.97 56.69 |
–1.2607 –1.2407 –1.233 |
2601.9 264.14 216.95 |
0.0054 0.0629 0.0657 |
0.0328 0.4054 0.4262 |
where the slopes of the cathodic and anodic polarization curves, referred to as Tafel slopes, are represented by bc and ba, respectively, measured in mV/dec; additionally, the polarization resistance is represented by Rp, measured in kΩ·c
| (12) |
where ΔE represents the polarization potential, and Δi represents the polarization current. It is evident that under similar potential conditions, the corrosion current decreases as the impedance increases. This relationship is supported by
The surface morphologies of materials before erosion-corrosion are shown in
Fig.6 Surface morphologies of materials before erosion-corrosion: (a) CN16; (b) YG6; (c) YG6X
Fig.7 Erosion-corrosion rate of CN16, YG6, and YG6X in different conditions: (a) alkaline solution; (b) sand-water mixture; (c) alkaline sand-water mixture
In dynamic corrosion with rapid flow of alkaline solution, corrosion products adhere to the corroded surface, creating a temporary barrier that prevents further contact between the solution and the material's fresh surface, thereby inhibiting corrosion. However, as the alkaline solution erodes the surface, dissolved corrosion products mix with the solution, allowing unrestricted direct contact and ongoing corrosion of the fresh surface.
Fig.8 Surface morphologies of CN16 (a), YG6 (b), and YG6X (c) after dynamic alkaline solution corrosion for 120 min
It also indicates that CN16 demonstrates the lowest volumetric loss rate. The resistance to abrasive particle wear is determined not only by hardness but also microstructure.
Fig.9 Surface morphologies of CN16 (a), YG6 (b), and YG6X (c) after erosion by sand-water mixture for 120 min
| SiO2+2NaOH=NaSiO3+H2O | (13) |
The metal binder phase in both the cermet and cemented carbides reacts with NaOH, resulting in the formation of corrosive oxidation products. These corrosion products initially cover the original surface and are subjected to intense erosion from the sand-water mixture. However, the adhesion between the corrosion products and the substrate is weak, leading to easy washout before completely covering the original surface. Consequently, they dissolve into the sand-water mixture. Moreover, the reaction between the metal binder phase and NaOH proceeds slowly. As a result, corrosion products exhibit restricted ability to inhibit the erosive wear and corrosion of the alkaline sand-water mixture. The removal of corrosion products signifies a loss of the binder phase, which contributes to the volumetric loss in both the cermet and cemented carbides.
Another factor contributing to surface damage and volumetric loss in cermet and cemented carbides is the delamination of the ceramic hard phase. In the erosion process, as abrasive sand particles impact the surface of materials, the high-speed impact forces vary in their probability of hitting the metal binder phase. Consequently, the metal binder phase experiences uneven stress, which leads to varying rates of chemical reactions. The regions experiencing higher stress are preferentially corroded, resulting in the expansion of the corrosion region. Upon destruction of the metal binder phase, corrosion pits form, intensifying the damage to the edges of the ceramic hard phase caused by abrasive sand particles. Moreover, the presence of corrosion pits increases the contact surface area between the material and the alkaline mixture, thus accelerating the corrosion process. As the abrasive sand particles impact and shatter the edges of the ceramic hard phase, the fragmented pieces are subsequently carried away by the running mixture, leading to volumetric loss of the material.
In the conditions of erosive and corrosive alkaline sand-water mixture, the metal binder phase plays a crucial role in preserving and delaying the destruction of the ceramic hard phase. The damage to cermet and cemented carbides primarily originates from the metal binder phase. Due to the significant difference in corrosion resistance between the metal binder phase and the ceramic hard phase, the metal binder phase firstly undergoes corrosion. Afterwards, the ceramic hard phase, enclosed by the metal binder phase, is directly exposed to the erosive mixture, and the surface exposure of the ceramic hard phase is increased in the eroding. Given the lower toughness of the ceramic hard phase, the exposed surfaces are vulnerable to crack formation and fragmentation upon impact by abrasive sand particles. Following the corrosion of the metal binder phase, the ceramic hard phase particles, previously bonded and supported by the metal binder phase, is detache
YG6 exhibits the highest rate of volumetric loss under the erosive-corrosive action of alkaline sand-water mixture, indicating its weakest resistance to erosion-corrosion. Although YG6 demonstrates better resistance to dynamic corrosion in alkaline solutions than YG6X, it shows notably weaker performance in resisting abrasion caused by sand-water mixture erosion than YG6X.
Fig.10 Surface morphologies of CN16 (a), YG6 (b), and YG6X (c) after erosion-corrosion by alkaline sand-water mixture
The surface of CN16 shows simultaneous traces of corrosion and erosion due to abrasive particle impact, as depicted in
By comparing material loss under various conditions such as dynamic alkaline solution corrosion, sand-water mixture abrasion, and erosion-corrosion by alkaline sand-water mixture, several conclusions can be made. When subjected to simple alkaline solution erosion (
From the morphology of erosion by sand-water mixture (
1) When exposed to alkaline solutions, the cermet exhibits passivation, which leads to a significant increase in oxygen content on the corroded surface. The fine-grained cemented carbides have higher corrosion rate, followed by the medium-grained cemented carbides. Notably, the corrosion rate of the cermet is approximately one order of magnitude lower than that of cemented carbides.
2) In the alkaline sand-water mixture, both the cermet and cemented carbides experience an increase in volume loss rate over time. The medium-grained cemented carbide exhibits the highest volume loss rate, followed by the fine-grained one. The cermet demonstrates the lowest volume loss rate. During the impacts by sand particles, cracks tend to penetrate the rim phase of the cermet, while the core phase remains relatively intact. In contrast, transgranular fracture is more common in medium-grained cemented carbides, and cracks tend to propagate along grain boundaries in fine-grained cemented carbides.
3) The abrasive effect of sand particles is the primary reason for degradation in the erosion-corrosion process of alkaline sand-water mixture. This process is not simply the combination of individual alkaline solution corrosion and sand-water mixture erosive wear. Instead, it involves mutual interaction and promotion of both erosive corrosion and wear, leading to an accelerated destructive process.
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